The present invention relates to a high frequency pulse transformer which has an air gap along a magnetic path by a ferrite core, and in particular, relates to such a transformer which is used for voltage conversion in a switching power source of a flyback converter type.
FIG. 5 shows a circuit diagram of a flyback converter type switching power source, in which the symbol B is a DC power source, T is a transformer, TR is a transistor, CT is a control circuit, D.sub.s is a diode, C.sub.s is a capacitor, and Z.sub.L is a load. The transistor TR switches ON and OFF repetitively according to the control of the control circuit CT. When the transistor TR is in ON state, the current flows from the DC power source B, through the primary winding N.sub.p of the transformer T, the transistor TR, to the DC power source B. Then, the energy is stored in the transformer T when the transistor TR is in ON state. That energy stored in the transformer T is proportional to 1/2LI.sup.2, where L is inductance of the primary winding N.sub.p, and I is current in the primary winding N.sub.p. Next, when the transistor is in OFF state, that energy is released to the load Z.sub.L through the diode D.sub.s and the capacitor C.sub.s which functions as a rectifier and a smoother. Therefore, the inductance L of the primary winding must have the proper value so that a lot of energy is stored in the transformer during the ON state of the transistor, although an usual transformer for voltage conversion has inductance as large as possible.
If the inductance L is too large, the current I would be small, and the stored energy would be small. Similarly, if the inductance is too small, the stored energy would also be small although the current I is large.
Therefore, it is preferable that a transformer for a flyback converter type switching power source has an air gap in a magnetic path for reducing or having proper inductance.
We first tried this kind of high frequency pulse transformer for switching power source, as shown in Fig. 3, in which a pair of E-shaped ferrite cores 1 abutted each other, an air gap G provided in a center leg of the core, and a bobbin with a winding on said center leg 2. The air gap G is essential in a transformer of a flyback converter type switching power source for storing electro-magnetic energy.
However, we found that the transformer as shown in Fig. 3 in which an air gap G is provided at the center portion of the center leg 2 has the disadvantage that leakage flux .phi. which leaks from said air gap G interlinks with a winding 3 as shown in FIG. 4, and the winding 3 is subject to eddy current loss. Therefore, the temperature of the winding 3 is raised partially. That disadvantage is serious when the capacity of the transformer is large, like a transformer for a switching power source.